U.S. patent application number 16/806767 was filed with the patent office on 2021-09-02 for method for enhanced bonding of thermoplastic composites.
The applicant listed for this patent is The Boeing Company. Invention is credited to Shantanu Bhattacharya, Om Prakash, Megha Sahu, Poonam Sundriyal.
Application Number | 20210269608 16/806767 |
Document ID | / |
Family ID | 1000004737106 |
Filed Date | 2021-09-02 |
United States Patent
Application |
20210269608 |
Kind Code |
A1 |
Sahu; Megha ; et
al. |
September 2, 2021 |
METHOD FOR ENHANCED BONDING OF THERMOPLASTIC COMPOSITES
Abstract
The present disclosure provides a method of enhancing the
shelf-life of an activated surface of a thermoplastic material,
including: coating at least a portion of a surface of the
thermoplastic material with at least one adhesion promoter to
provide a coated surface; and treating the coated surface with
plasma to provide the activated surface of the thermoplastic
material; wherein the activated surface has a contact angle in the
range of from about 0 to about 40.degree.; and wherein the presence
of the at least one adhesion promoter is effective to maintain the
contact angle in the range of from about 0 to about 40.degree. for
a time of about 10 days or greater.
Inventors: |
Sahu; Megha; (Bangalore,
IN) ; Prakash; Om; (Bangalore, IN) ;
Bhattacharya; Shantanu; (Kanpur, IN) ; Sundriyal;
Poonam; (Kanpur, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Boeing Company |
Chicago |
IL |
US |
|
|
Family ID: |
1000004737106 |
Appl. No.: |
16/806767 |
Filed: |
March 2, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C08J 7/043 20200101;
C08J 2371/00 20130101; C08J 2463/00 20130101; B05D 3/148 20130101;
C08J 5/124 20130101 |
International
Class: |
C08J 7/043 20060101
C08J007/043; B05D 3/14 20060101 B05D003/14; C08J 5/12 20060101
C08J005/12 |
Claims
1. A method of enhancing the shelf-life of an activated surface of
a thermoplastic material, comprising: coating at least a portion of
a surface of the thermoplastic material with at least one adhesion
promoter to provide a coated surface; and treating the coated
surface with plasma to provide the activated surface of the
thermoplastic material; wherein the activated surface has a contact
angle in the range of from about 0 to about 40.degree.; and wherein
the presence of the at least one adhesion promoter is effective to
maintain the contact angle in the range of from about 0 to about
40.degree. for a time of about 10 days or greater.
2. The method of claim 1, wherein the at least one adhesion
promoter is selected from the group consisting of PEG-silane,
polyvinyl alcohol (PVA), 3-Glycidoxypropyl methyldimethoxysilane,
3-Chloropropyltrimethoxysilane, vinyltriethoxysilane, zirconium
acetylacetonate, and combinations thereof.
3. The method of claim 1, wherein the at least one adhesion
promoter is PEG-silane.
4. The method of claim 1, wherein the at least one adhesion
promoter is a silane.
5. The method of claim 1, wherein the presence of the at least one
adhesion promoter is effective to maintain the contact angle in the
range of from about 0 to about 40.degree. for a time of about 15
days or greater.
6. The method of claim 1, wherein the presence of the at least one
adhesion promoter is effective to maintain the contact angle in the
range of from about 0 to about 40.degree. for a time of about 25
days or greater.
7. The method of claim 1, wherein the coated surface has an
adhesion promoter coating layer having a thickness of from about 2
to about 12 microns.
8. The method of claim 1, wherein the thermoplastic material is
polyether ether ketone.
9. A thermoplastic material having at least one activated surface
prepared according to the method of claim 1.
10. A method of forming a thermoplastic composite structure,
comprising: providing a first thermoplastic part and a second
thermoplastic part; coating at least a portion of a surface of the
first thermoplastic part with at least one adhesion promoter to
provide a coated surface; treating the coated surface with plasma
to provide an activated surface; and bonding the first
thermoplastic part and the second thermoplastic part at the
activated surface with at least one adhesive to form the
thermoplastic composite structure; wherein the activated surface
has a contact angle in the range of from about 0 to about
40.degree.; and wherein the presence of the at least one adhesion
promoter is effective to maintain the contact angle in the range of
from about 0 to about 40.degree. for a time of about 10 days or
greater.
11. The method of claim 10, wherein the at least one adhesion
promoter is selected from the group consisting of PEG-silane,
polyvinyl alcohol (PVA), 3-Glycidoxypropyl methyldimethoxysilane,
3-Chloropropyltrimethoxysilane, vinyltriethoxysilane, zirconium
acetylacetonate, and combinations thereof.
12. The method of claim 10, wherein the thermoplastic composite
structure has a bond strength of from about 20 to about 30 MPa.
13. The method of claim 10, wherein the at least one adhesive
material is epoxy.
14. The method of claim 10, further comprising: coating at least a
portion of a surface of the second thermoplastic part with at least
one second adhesion promoter to provide a coated surface; treating
the coated surface of the second thermoplastic part with plasma to
provide a second activated surface; and bonding the first
thermoplastic part and the second thermoplastic part to create a
bond, such that the activated surface is adjacent to and facing the
second activated surface at the bond; wherein the second activated
surface has a second contact angle in the range of from about 0 to
about 40.degree.; and wherein the presence of the at least one
second adhesion promoter is effective to maintain the second
contact angle in the range of from about 0 to about 40.degree. for
a time of about 10 days or greater.
15. The method of claim 10, wherein the first thermoplastic part
comprises polyether ether ketone.
16. The method of claim 10, wherein the second thermoplastic part
comprises polyether ether ketone.
17. A thermoplastic composite structure formed according to the
method of claim 10.
18. The thermoplastic composite structure of claim 17, wherein the
thermoplastic composite structure is configured for use in an
aerospace vehicle.
Description
BACKGROUND
Field of the Disclosure
[0001] The present disclosure relates to improvement of adhesion in
thermoplastics, in particular, to thermoplastic materials
exhibiting enhanced bonding characteristics and methods for
providing enhanced bonding of thermoplastic composite
materials.
Description of Related Art
[0002] Thermoplastic composite materials are used in a wide variety
of applications. For example, thermoplastic composite materials are
employed within aircrafts. Some example applications of
thermoplastic composites within aerospace devices and aerospace
vehicles such airplanes, rotocraft, drones, and other aircraft
include the following: framing, flooring, and seats; wings and wing
parts, control surfaces, fuselage panels, engine parts, and other
like components and parts. It is desired to have efficient joining
of composite structures for cost-effective manufacture of modern
aerospace components and aerospace vehicles.
[0003] Thermoplastics pose challenges relative to adhesive bonding.
The bonding quality greatly depends on surface energies of the
thermoplastic materials, which is related to wetting angle/contact
angle of the surface. Bonding effectiveness of a thermoplastic
material can be enhanced by altering the surface of the
thermoplastic material to provide a lower contact angle. Therefore,
conventional bonding methods require specialized surface
preparation and cleaning is necessary to obtain increased
bonding.
[0004] Plasma treatment of thermoplastic surfaces is one known
pre-treatment before adhesive bonding is applied. Plasma cleaning
is the removal of impurities and contaminants from surfaces through
the use of an energetic plasma or dielectric barrier discharge
(DBD) plasma created from gaseous species. Gases such as argon and
oxygen, as well as mixtures such as CO.sub.2, O, ozone, other air
components, and hydrogen/nitrogen are typically used. The plasma is
created by using high frequency voltages to ionize the low pressure
gas, although atmospheric pressure plasmas are also common. The
plasma can then be used to interact with (i.e., clean) any surface
placed in the plasma.
[0005] The use of plasma treatment as a precursor to adhesive
bonding currently has issues related to shelf-life. In particular,
samples subjected to conventional plasma pre-treatment processes
have limited shelf-life with regard to retaining enhanced surface
activity over a period of time, typically two weeks. The limited
shelf-life prevents the use of plasma treatment as a widely used
industrial process in manufacturing of thermoplastic composites and
assemblies.
[0006] It would be advantageous to provide methods for enhancing
the bonding capabilities of thermoplastic materials. In particular,
it would be advantageous to provide pre-treatment processes for
thermoplastic polymers which extend the shelf-life of
plasma-treated thermoplastic materials (i.e., the shelf-life
available for subsequent processes after the application of
plasma).
SUMMARY
[0007] The disclosure provides thermoplastic materials exhibiting
an enhanced shelf-life for subsequent bonding processes after the
thermoplastic material has been treated with plasma. Methods for
enhancing the shelf-life for subsequent bonding processes after a
thermoplastic material has been treated with plasma are also
provided herein.
[0008] Treating the surface of a thermoplastic material with plasma
can enhance the bonding of the thermoplastic material in composite
structures. However, such pre-treated samples have limited
shelf-life in regard to retaining the enhanced surface activity
over a period of time, typically two weeks. The current disclosure
demonstrates a capping mechanism of plasma-treated thermoplastic
surfaces with a layer of thin adhesion promoter coating, which
restricts the plasma treated surface from recovery processes and
increases the shelf-life of such polymers.
[0009] The present disclosure includes, without limitation the
following embodiments.
[0010] Embodiment 1: A method of enhancing the shelf-life of an
activated surface of a thermoplastic material, comprising: coating
at least a portion of a surface of the thermoplastic material with
at least one adhesion promoter to provide a coated surface; and
treating the coated surface with plasma to provide the activated
surface of the thermoplastic material; wherein the activated
surface has a contact angle in the range of from about 0 to about
40.degree.; and wherein the presence of the at least one adhesion
promoter is effective to maintain the contact angle in the range of
from about 0 to about 40.degree. for a time of about 10 days or
greater.
[0011] Embodiment 2: The method of the preceding embodiment,
wherein the at least one adhesion promoter is selected from the
group consisting of: PEG-silane, polyvinyl alcohol (PVA),
3-Glycidoxypropyl methyldimethoxysilane,
3-Chloropropyltrimethoxysilane, vinyltriethoxysilane, zirconium
acetylacetonate, and combinations thereof.
[0012] Embodiment 3: The method of any preceding embodiment,
wherein the at least one adhesion promoter is PEG-silane.
[0013] Embodiment 4: The method of any preceding embodiment,
wherein the presence of the at least one adhesion promoter is
effective to maintain the contact angle in the range of from about
0 to about 40.degree. for a time of about 15 days or greater.
[0014] Embodiment 5: The method of any preceding embodiment,
wherein the presence of the at least one adhesion promoter is
effective to maintain the contact angle in the range of from about
0 to about 40.degree. for a time of about 25 days or greater.
[0015] Embodiment 6: The method of any preceding embodiment,
wherein the coated surface has an adhesion promoter coating layer
having a thickness of from about 2 to about 12 microns.
[0016] Embodiment 7: The method of any preceding embodiment,
wherein the thermoplastic polymer material is polyether ether
ketone.
[0017] Embodiment 8: A thermoplastic polymer material having at
least one activated surface prepared according to the method of any
preceding embodiment.
[0018] Embodiment 9: A method of forming a thermoplastic composite
structure, comprising: providing a first thermoplastic part and a
second thermoplastic part; coating at least a portion of a surface
of the first thermoplastic part with at least one adhesion promoter
to provide a coated surface; treating the coated surface with
plasma to provide an activated surface; and bonding the first
thermoplastic part and the second thermoplastic part at the
activated surface with at least one adhesive to form the
thermoplastic composite structure; wherein the activated surface
has a contact angle in the range of from about 0 to about
40.degree.; and wherein the presence of the at least one adhesion
promoter is effective to maintain the contact angle in the range of
from about 0 to about 40.degree. for a time of about 10 days or
greater.
[0019] Embodiment 10: The method of any preceding embodiment,
wherein the at least one adhesion promoter is selected from the
group consisting of PEG-silane, polyvinyl alcohol (PVA),
3-Glycidoxypropyl methyldimethoxysilane,
3-Chloropropyltrimethoxysilane, vinyltriethoxysilane, zirconium
acetylacetonate, and like materials, and combinations thereof.
[0020] Embodiment 11: The method of any preceding embodiment,
wherein the thermoplastic composite structure has a bond strength
of from about 20 to about 30 MPa.
[0021] Embodiment 12: The method of any preceding embodiment,
wherein the at least one adhesive material is epoxy.
[0022] Embodiment 13: The method of any preceding embodiment,
further comprising: coating at least a portion of a surface of the
second thermoplastic part with at least one adhesion promoter to
provide a coated surface; treating the coated surface with plasma
to provide a second activated surface; and bonding the first
thermoplastic part and the second thermoplastic part such that the
activated surface is adjacent to and facing the second activated
surface at the bond; wherein the second activated surface has a
second contact angle in the range of from about 0 to about
40.degree.; and wherein the presence of the at least one adhesion
promoter is effective to maintain the second contact angle in the
range of from about 0 to about 40.degree. for a time of about 10
days or greater.
[0023] Embodiment 14: The method of any preceding embodiment,
wherein the first thermoplastic part comprises polyether ether
ketone.
[0024] Embodiment 15: The method of any preceding embodiment,
wherein the second thermoplastic part comprises polyether ether
ketone.
[0025] Embodiment 16: The method of any preceding embodiment,
wherein the at least one adhesion promoter is a silane.
[0026] Embodiment 17: A thermoplastic structure formed according to
the method of any preceding embodiment.
[0027] Embodiment 18: The thermoplastic structure of any preceding
embodiment, wherein the thermoplastic composite structure is
configured for use in an aircraft.
[0028] These and other features, aspects, and advantages of the
present disclosure will be apparent from a reading of the following
detailed description together with the accompanying drawings, which
are briefly described below. The present disclosure includes any
combination of two, three, four, or more embodiments, features, or
elements set forth in this disclosure, regardless of whether such
embodiments, features, or elements are expressly combined or
otherwise recited in a specific embodiment description herein. This
disclosure is intended to be read holistically such that any
separable features or elements of the disclosure, in any of its
aspects and embodiments, should be viewed as intended, namely to be
combinable, unless the context of the disclosure clearly dictates
otherwise.
[0029] It will be appreciated that the summary herein is provided
merely for purposes of summarizing some example aspects so as to
provide a basic understanding of the disclosure. As such, it will
be appreciated that the above described example embodiments are
merely examples and should not be construed to narrow the scope or
spirit of the disclosure in any way. It will be appreciated that
the scope of the disclosure encompasses many potential embodiments,
some of which will be further described below, in addition to those
herein summarized. Further, other aspects and advantages of such
embodiments disclosed herein will become apparent from the
following detailed description taken in conjunction with the
accompanying drawings which illustrate, by way of example, the
principles of the described aspects.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] Having thus described the disclosure in general terms,
reference will now be made to the accompanying drawings, which are
not necessarily drawn to scale, and wherein:
[0031] FIG. 1 is a block diagram of the mechanism involved in
embodiments of the thermoplastic treatment methods described
herein;
[0032] FIG. 2 illustrates the surface contact angle of an untreated
PEEK sample;
[0033] FIGS. 3(a)-3(h) illustrate the time dependent change in the
contact angles of the: (a-b) plasma treated control PEEK sample and
(c-h) PEG-silane and plasma treated inventive PEEK sample according
to an embodiment of the present disclosure;
[0034] FIG. 4 is an ATR-FTIR spectra of a PEG-silane and plasma
treated PEEK sample according to an embodiment of the present
disclosure at different time instants;
[0035] FIG. 5 shows comparative Thermal Gravimetric Analysis (TGA)
plots for a control PEEK sample and a PEG-silane coated PEEK sample
according to an embodiment of the present disclosure;
[0036] FIG. 6 is a stress-strain diagram of a PTPSP (Plasma treated
and PEG-Silane coated PEEK)-Epoxy-PTPSP joint and the effect of
water/ethanol immersion on the stress-strain curves;
[0037] FIGS. 7(a) and 7(b) are stress-strain diagrams showing the
bond strength of different PEEK samples; and
[0038] FIGS. 8(a)-(d) show FESEM images of different PEEK
samples.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0039] The present disclosure now will be described more fully
hereinafter with reference to the accompanying drawings, in which
some, but not all aspects of the disclosure are shown. Indeed, the
disclosure may be embodied in many different forms and should not
be construed as limited to the aspects set forth herein; rather,
these aspects are provided so that this disclosure will satisfy
applicable legal requirements. Like numbers refer to like elements
throughout.
[0040] Thermoplastics are difficult to join using adhesive bonding
alone. Plasma treatment technologies are used to activate (i.e.,
clean) the surface of a thermoplastic material such that the
bonding characteristics of the thermoplastic material are enhanced.
However as noted herein above, the activated surface retains the
activity for a very limited time and there is a need to extend the
shelf-life of the activated surface. Without being limited by
theory, extending the shelf-life of the plasma treatment, which
provides an activated surface of the treated material, can allow
for better alignment with time scales relevant for industrial
manufacturing practice. For example, conventional plasma treated
samples should be used (i.e., subjected to any desired processes
such as bonding following the plasma treatment) within 7-10 days,
thereby limiting the application of the plasma treatment. As
described in more detail herein below, the present disclosure
provides thermoplastic materials exhibiting an activated surface of
10 days or longer, methods for extending the shelf-life of a
plasma-activated surface, and methods for forming composite
structures exhibiting an enhanced bonding strength.
[0041] As noted herein above, the bonding characteristics of a
thermoplastic material are related to the contact angle of the
surface of the thermoplastic material. Bonding effectiveness of a
thermoplastic material can be enhanced by altering the surface of
the thermoplastic material to provide a lower contact angle. Low
contact angles are generally desired for good bonding strength.
Plasma treatment is effective to lower the surface contact angle of
a thermoplastic material. Methods of the present disclosure can
include any plasma treatment process known in the art to provide an
activated surface of a thermoplastic material. See, e.g., inagaki,
N., Tasaka, S., Horiuchi, T., & Suyama, R. (1998). Surface
modification of poly (aryl ether ether ketone) film by remote
oxygen plasma. Journal of applied polymer science, 68(2), 271-279;
Larson, B. J., Gillmor; S. D., Braun, J. M.; Cruz-Barba, L. E.,
Savage, D. E., Denes, F. S., & Lagally, M. G. (2013), Long-Term
Reduction in Poly (dimethylsiloxane) Surface Hydrophobicity via
Cold-Plasma Treatments. Langmuir, 29(42), 12990-129% Ba, O. M.,
Marmey, P., Anselme, K., Duncan, A. C.; & Ponche; A. (2016).
Surface composition XI'S analysis of a plasma treated polystyrene:
Evolution over long storage periods. Colloids and Surfaces B:
Biointerfaces, 145, 1-7; Kim, Y. Provine, J., Walch, S. P. Park,
J., Phuthong, W., Dadlani, A., L., & Prinz, F. B. (2016).
Plasma-enhanced atomic layer deposition of SiN.sub. AlN composites
for ultra-low wet etch rates in hydrofluoric acid. ACS applied
materials & interfaces, 8(27), 17599-17605; Guruvenket, S.,
Rao, G. M., Komath, M., & Raichur, A. M. (2004). Plasma surface
modification of polystyrene and polyethylene. Applied Surface
Science, 236(1-4), 278-284; and Sanchis, M. R., Calvo, O.,
Fenollar, O., Garcia, D., & Balart, R. (2008). Characterization
of the surface changes and the aging effects of low-pressure
nitrogen plasma treatment in a polyurethane film. Polymer testing,
27(1), 75-83; each of which are herein incorporated by
reference.
[0042] Good bonding strength of a thermoplastic material can be
achieved for thermoplastic materials having a surface contact angle
in the range of from about 0 to about 40.degree.. In various
embodiments of the present disclosure, the activated surface of a
plasma-treated thermoplastic material can have a surface contact
angle in the range of from about 0 to about 40.degree., from about
5 to about 35.degree., from about 10 to about 33.degree., from
about 10 to about 31.degree., or from about 10 to about 30.degree..
In some embodiments, the activated surface of a plasma-treated
thermoplastic material can have a surface contact angle of about
40.degree. or less, about 35.degree. or less, about 33.degree. or
less, about 31.degree. or less, or about 30.degree. or less.
[0043] FIG. 2, for example, illustrates the surface contact angle
of an untreated polyether ether ketone (PEEK) sample. The untreated
PEEK surface is mildly hydrophobic (contact angle 70 degrees). As
illustrated in FIG. 3, and described in Example 1 below, a
plasma-treated PEEK surface exhibits a contact angle of about
10.degree., but the same degrades back to contact angle of
70.degree. in 7 days time.
[0044] It was discovered that thermoplastic materials treated with
an adhesion promoter prior to the plasma treatment exhibit an
extended shelf-life of the plasma-activated surface. As illustrated
in FIG. 3 and described in Example 1 below, for example, a
plasma-treated PEEK sample that was first coated with a thin layer
of PEG-silane adhesion promoter demonstrated a contact angle of
about 30.degree. for a duration as long as 7 weeks. For example,
FIG. 3 shows the contact angle of a sample prepared according to an
embodiment of the present disclosure staying at 29.6.degree. after
14 days (FIG. 3 (e)), contact angle of 30.3.degree. after 21 days
(FIG. 3 (f)), contact angle of 31.5.degree. after 35 days (FIG. 3
(g)), and contact angle of 33.2.degree. after 48 days (FIG. 3(h)).
As used herein, an "adhesion promoter" refers to materials that can
serve as a coating layer with promotion for interlayer adhesion of
thermoplastic materials. In some embodiments, the adhesion promoter
can have brush polymer-like properties, therefore having a tendency
to form a dense structure at the interface of the surface of the
thermoplastic material to be coated and thereby block the release
of the short and long chain oligomers from the bulk of the coated
thermoplastic polymer material. The surface adhesion depends on the
functional groups present at the surface. The adhesion promoter
promotes adhesion due to the steric hindrance provided by its
structure and thus, retards the decay in the effects of plasma
treatment. It is noted that the description and the examples of the
present disclosure refer to treatment processes applied to
polyether ether ketone (PEEK), however, the present disclosure is
not intended to be limited to PEEK and the methods described herein
are applicable to all thermoplastic materials and any additional
materials capable of being subjected to a plasma treatment process.
According to various embodiments, the modification of the surface
of the thermoplastic polymer includes a chemical capping layer of
an adhesion promoter (e.g., polyethylene glycol (PEG)-silane),
which extends the time horizon for surface recovery of plasma
exposed thermoplastic material surfaces.
[0045] FIG. 1 is a block diagram illustrating the mechanism
involved in preparing an example thermoplastic material of the
present disclosure. Treatment of a PEEK surface with plasma and a
PEG-silane adhesion promoter enhances activation of the surface.
Specifically, plasma enhances the availability of OH groups for
bonding. The PEG-silane extends the availability of the OH groups
to bond for a longer time by capping those groups. The long term
stability of the plasma modified surfaces improves adhesion aspects
with epoxy inter-layers for bond assemblies between thermoplastic
polymer composites.
[0046] FIGS. 8(a)-(d) show FESEM images of different PEEK samples.
FIG. 8(a) is as FESEM image of an untreated PEEK sample. FIG. 8(b)
is an FESEM image of a plasma-treated PEEK sample. FIG. 8(c) is an
FESEM image of a PEG-silane coated PEEK sample. FIG. 8(d) is an
FESEM image of a plasma-treated and PEG-silane coated PEEK sample.
FTIR spectra of the different PEEK samples show that the --OH and
N--H groups are significantly increased after plasma treatment.
With time, the --OH and N--H groups decrease in the plasma-treated
PEEK sample (FIG. 8(b)) at a higher rate than in the plasma-treated
and PEG-silane coated PEEK sample (FIG. 8(d)). FTIR spectra of the
different PEEK samples also show that --CH groups are decreased at
the surface following plasma treatment. With time, the --CH groups
increased in the plasma-treated PEEK sample (FIG. 8(b)) at a higher
rate than in the plasma-treated and PEG-silane coated PEEK sample
(FIG. 8(d)).
[0047] In various embodiments of the present disclosure, the
adhesion promoter can be selected from the group consisting of
polyethylene glycol (PEG)-silane, polyvinyl alcohol (PVA),
3-Glycidoxypropyl methyldimethoxysilane,
3-Chloropropyltrimethoxysilane, vinyltriethoxysilane, zirconium
acetylacetonate, and combinations thereof. In certain embodiments,
the methods of the present disclosure include treating (e.g.,
coating) the surface of a thermoplastic polymer with PEG-silane
prior to a subsequent plasma treatment.
[0048] In various embodiments, the presence of at least one
adhesion promoter on a surface of a thermoplastic material
subjected to a plasma treatment process is effective to maintain
the contact angle of the plasma-treated thermoplastic material in
the range of from about 0 to about 40.degree., from about 5 to
about 35.degree., from about 10 to about 33.degree., or from about
10 to about 31.degree. for a time of about 10 days or greater,
about 15 days or greater, about 20 days or greater, about 25 days
or greater, about 30 days or greater, about 35 days or greater,
about 40 days or greater, or about 45 days or greater.
[0049] In various embodiments, the adhesion promoter coating can be
applied to a surface of a thermoplastic polymer through a spin
coating, dip coating, and/or spray coating process. In certain
embodiments, the adhesion promoter can be applied to the surface of
a thermoplastic polymer using a blade coating process and a vacuum
desiccator. Correct thermal treatment can be used to ensure
stability of the coating (e.g., drying the coatings for a period of
6-24 hours at an elevated temperature such as about 70-100.degree.
C.). In some embodiments, the adhesion promoter coating can be
dried for a period of about 4-48 hours, or about 6-24 hours, or
about 8-12 hours. In various embodiments, the adhesion promoter
coating can be dried at a temperature of at least about 70.degree.
C., at least about 80.degree. C., at least about 90.degree. C., or
at least about 100.degree. C. The drying time and temperature for
the correct thermal treatment can depend on, for example, the
adhesion promoter selected, the thermoplastic material selected,
and the thickness of the adhesion promoter coating.
[0050] In some embodiments, the thickness of the adhesion promoter
film/coating on the surface of thermoplastic polymer after thermal
treatment can be in the range of from about 2 to about 12 microns,
from about 3 to about 10 microns, or from about 4 to about 10
microns. Detection of the adhesion promoter coating is possible
through spectroscopic characterization of surfaces, for
example.
[0051] The PEG-Silane coating can be applied over the Peek surfaces
by using blade coating and vacuum desiccator. The films can also be
dried for up to about 8 hr in an oven at a temperature of about
80.degree. C., for example.
[0052] Thermoplastic materials prepared according to the present
disclosure (i.e., coated with an adhesion promoter and then treated
with plasma) exhibit an improved bond strength capability.
Thermoplastic materials of the present disclosure can be used to
prepare composite structures using any bonding methods known in the
art. See, e.g., the processes described in Nash, N. H., Young, T.
M., Mc Grail, P. T., & Stanley, W. F. (2015). Inclusion of a
thermoplastic phase to improve impact and post-impact performances
of carbon fibre reinforced thermosetting composites--A review.
Materials & Design, 85, 582-597; Basturk S B. Development and
mechanical characterization of anti-blast sandwich composites for
explosive effect. PhD Thesis, Izmir Institute of Technology,
Turkey, 2012; Ebnesajjad C. Theories of adhesion. In: Ebnesajjad C,
editor. Surface treatment of materials for adhesive bonding.
Elsevier Science; 2013; Petrie E M. Theories of adhesion. In:
Petrie E M, editor. Handbook of adhesives and sealants.
McGraw-Hill; 2000; Villegas, I. F., & van Moorleghem, R.
(2018). Ultrasonic welding of carbon/epoxy and carbon/PEEK
composites through a PEI thermoplastic coupling layer. Composites
Part A: Applied Science and Manufacturing, 109, 75-83; Xie, L.,
Liu, H., Wu, W., Abliz, D., Duan, Y., & Li, D. (2016). Fusion
bonding of thermosets composite structures with thermoplastic
binder co-cure and prepare interlayer in electrical resistance
welding. Materials & Design, 98, 143-149; Shi, H., Sinke, J.,
& Benedictus, R. (2017). Surface modification of PEEK by UV
irradiation for direct co-curing with carbon fibre reinforced epoxy
prepregs. International Journal of Adhesion and Adhesives, 73,
51-57; Benatar, A. and Gutowski, T. G. (1986). Methods for Fusion
Bonding Thermoplastic Composites, SAMPE Quarterly. 18(1): 35-42;
Border, J. and Salas, R. (1989). Induction Heated Joining of
Thermoplastic Composites Without Metal Susceptors, 34th
International SAMPE Symposium, pp. 2569-2578; Ageorges, C., Ye, L.
and Hou, M. (2000). Experimental Investigation of the Resistance
Welding for Thermoplastic-Matrix Composites. Part II: Optimum
Processing Window and Mechanical Performance, Composites Science
and Technology. 60: 1191-1202; S. Deng et al./Composites: Part A 68
(2015) 121-132 `Thermoplastic--epoxy interactions and their
potential applications in joining composite structures--A review`;
each of which are herein incorporated by reference.
[0053] In some embodiments, a thermoplastic material coated with an
adhesion promoter and then treated with plasma can be used to form
a composite structure having a bond strength of from about 20 to
about 30 MPa, or from about 22 to about 27 MPa. It is noted that
the current disclosure is applicable for all types of adhesive
bonds. As such, bond strength can vary based on different
parameters (e.g., materials being bonded, adhesive used to bond,
etc.) and therefore applicable bond strengths may be outside of
this range. In various embodiments, a thermoplastic material coated
with an adhesion promoter and then treated with plasma can show an
increase in bond strength, as compared to an untreated
thermoplastic material, of at least about 10 MPa, at least about 15
MPa, or at least about 20 MPa.
[0054] Methods of preparing a thermoplastic material exhibiting an
improved shelf-life of an activated surface are also provided
herein. The methods can include coating at least a portion of a
surface of the thermoplastic material with at least one adhesion
promoter to provide a coated surface, and treating the coated
surface with plasma to provide the activated surface of the
thermoplastic material. As described above, an activated surface
refers to a surface that provides enhanced adhesive bonding
capabilities. The activated surface can be defined by a surface
contact angle in the range of from about 0 to about 40.degree.. In
various embodiments of the methods described herein, the presence
of the at least one adhesion promoter is effective to maintain the
contact angle in the range of from about 0 to about 40.degree. for
a time of about 10 days or greater, about 15 days or greater, about
20 days or greater, about 30 days or greater, about 40 days or
greater, or about 45 days or greater.
[0055] A method of forming a thermoplastic composite structure is
also provided herein. The method can include providing a first
thermoplastic part and a second thermoplastic part, coating at
least a portion of a surface of the first thermoplastic part with
at least one adhesion promoter to provide a coated surface,
treating the coated surface with plasma to provide an activated
surface, and bonding the first thermoplastic part and the second
thermoplastic part at the activated surface with at least one
adhesive to form the thermoplastic composite structure. Examples of
adhesives include, but are not limited to, epoxy, polyurethane,
acrylic, and like adhesives, and mixtures of different types of
adhesives. It is noted that one or both of the first and
thermoplastic part can be treated to have an activated surface. As
such, the method of forming a thermoplastic composite structure can
further include coating at least a portion of a surface of the
second thermoplastic part with at least one adhesion promoter to
provide a coated surface, and treating the coated surface with
plasma to provide a second activated surface. The first
thermoplastic part can be adhesively bound to the second
thermoplastic part such that the activated surface of the first
thermoplastic part is adjacent to and facing the activated surface
of the second thermoplastic part. In other words, the adhesive
material can be applied between two activated surfaces. As
described herein above, the one or more activated surfaces can each
have a contact angle in the range of from about 0 to about
40.degree. and the presence of the at least one adhesion promoter
can be effective to maintain the contact angle in the range of from
about 0 to about 40.degree. for a time of about 10 days or greater,
about 15 days or greater, about 20 days or greater, about 30 days
or greater, about 40 days or greater, or about 45 days or greater.
Furthermore, the thermoplastic composite structure can have a bond
strength of from about 20 to about 30 MPa. The thermoplastic
composite structure can be configured for use in an aerospace
vehicle.
EXAMPLES
[0056] The present disclosure can be more fully illustrated by the
following examples, which are set forth to illustrate some
embodiments of the present disclosure and are not to be construed
as limiting thereof. All weight percentages are expressed on a dry
weight basis, meaning water content is excluded, unless otherwise
indicated.
Example 1
[0057] The contact angles of a control PEEK sample and an inventive
PEEK sample subjected to a plasma treatment process were measured
over time.
[0058] The inventive PEEK sample was first coated with a coating
film of PEG-silane. A blade coating process was used to apply the
coating layer over the inventive PEEK surface and the layer was
subsequently dried at 80.degree. C. for 8 hours. The thickness of
the coating layer was 4 to 10 .mu.m.
[0059] The control PEEK sample (i.e., having no PEG-silane coating)
and the inventive PEEK sample (i.e., having a PEG-silane coating)
were subjected to a nitrogen plasma treatment process. The
parameters of the plasma treatment powers were the following: 20
Watt power, 100 m-torr pressure, and 60 sec duration. The plasma
parameters are specific to nitrogen. However, similar chemistries
can be easily induced on the thermoplastic surface with gases such
as ammonia, oxygen, hydrogen, etc. by taking these gases into
plasma states where the amide and hydroxide bonds on the surface
can be initiated, thereby providing an increase the bond strength.
The parametric optimization for the different gases can be
independently carried out according to conventional plasma
treatment methods.
[0060] FIGS. 3(a)-(h) show the time dependent change in the contact
angles of the: (a-b) plasma-treated control PEEK sample and (c-h)
the PEG-silane and plasma-treated inventive PEEK sample. FIG. 4 is
an ATR-FTIR spectra of the PEG-silane and plasma treated inventive
PEEK sample at different time instants. This figure shows the
quantitative difference in the presence of various bonds due to
plasma treatment. The PEG-silane coating increases the --OH
functional groups on the PEEK surface and retains to the long term
surface modification durability due to the steric hindrance effect
emerged by its brush-like structure. As shown in FIG. 3(a), the
contact angle of the control PEEK sample directly after the plasma
treatment (i.e., time=0 days) was 7.degree.. As shown in FIG. 3(b),
the contact angle of the control PEEK sample 7 days after the
plasma treatment was 74.4.degree.. As shown in FIG. 3(c), the
contact angle of the inventive PEEK sample directly after the
plasma treatment (i.e., time=0 days) was 28.1.degree.. As shown in
FIGS. 3(d)-3(h), the contact angle of the inventive PEEK sample did
not significantly change over time. As shown in FIG. 3(h), the
contact angle of the control PEEK sample 48 days after the plasma
treatment was 33.2.degree., which is still within the contact angle
range representative of an activated surface (i.e., a contact angle
in the range of about 0 to about 40.degree.). The PEG-silane
coating improves the durability of the modified PEEK surfaces due
to steric hindrance provided from its brush-like nature. However,
the PEG-silane coating increases the initial contact angles of the
PEEK surfaces, which, without being limited by theory, may be due
to the change in the surface roughness of the coated surface.
[0061] Accordingly, it is clear that the PEG-silane coating layer
applied to the inventive PEEK sample prior to plasma treatment
extended the time which the plasma-treated material maintained an
activated surface (i.e., a surface having a contact angle in the
range of 0-40.degree.). Without being limited by theory, it is
believed that the adhesion promoter monolayer alters the surface
chemistry relative to plasma activation and helps retain
hydrophilicity of the surface for an extended duration of time.
Example 2
[0062] The effects of temperature on the samples prepared according
to Example 1 above were measured.
[0063] FIG. 5 shows comparative Thermal Gravimetric Analysis (TGA)
plots for the control PEEK and PEG-silane coated inventive PEEK
samples. The TGA was performed on the control PEEK and inventive
PEG-Silane coated PEEK samples in the temperature range of 0 to
900.degree. C. FIG. 5 shows minor difference between the thermal
degradation of both the samples. At 150.degree. C., the PEEK
control sample displays a weight of 99.2%. At 150.degree. C., the
inventive PEG-silane coated PEEK sample displays a weight of 98.8%.
A slightly more visible change of weight loss was observed for both
samples at 500.degree. C. with weight retention of 97.09% for the
control PEEK sample, and 95.6% for the inventive PEG-silane coated
PEEK sample. These results confirm that the PEG-silane coating has
only a minor effect on the thermal properties of the PEEK material.
Thus, the PEG-silane coating can be used for temperature sensitive
applications.
Example 3
[0064] The effects of solvents on the bond strength of the samples
prepared according to Example 1 above were measured.
[0065] Plasma treated and PEG-Silane coated PEEK (PTPSP) samples
were prepared according to Example 1 above. Two PTPSP samples were
bonded with epoxy. Three separate PTPSP-Epoxy-PTPSP composite
structures were formed. One PTPSP-Epoxy-PTPSP composite structure
was immersed in water for 6 hours. A second PTPSP-Epoxy-PTPSP
composite structure was immersed in ethanol for 6 hours. A third
PTPSP-Epoxy-PTPSP composite structure was not immersed in a
liquid.
[0066] FIG. 6 is a stress-strain diagram of a PTPSP-Epoxy-PTPSP
joint and the effect of water/ethanol immersion on the
stress-strain curves. The figure shows that there was no
significant change in the joint strength of the samples due to
water/ethanol immersion.
Example 4
[0067] The bonding strength of different PEEK samples was
evaluated.
[0068] Four different types of samples were prepared: (1) an
untreated PEEK sample (P); (2) a plasma-treated PEEK sample (PTP);
(3) a PEG-silane treated PEEK sample (PSP); and (4) a
plasma-treated and PEG-silane treated PEEK sample (PTPSP). The
total length of each sample was 101.6.+-.0.0254+L mm (where L is
the joint length). The thickness of each sample was 4.76 mm. The
width of each sample was 24.5.+-.0.0254 mm. Two samples of the same
type were bonded together using epoxy. A lap shear test was
performed on the different contacting surfaces based on the ASTM D
3163 standards.
[0069] FIG. 7(a) is a stress-strain diagram showing the bond
strength of different PEEK samples. FIG. 7(b) is a stress-strain
diagram showing the bond strength of different PEEK samples
immediately after plasma exposure and also 7 days after plasma
exposure. As illustrated in FIG. 7(a), the P-epoxy-P composite
structure exhibited a bond strength of 9.04 MPa. The PTP-epoxy-PTP
composite structure exhibited a bond strength of 24.86 MPa. The
PSP-epoxy-PSP composite structure exhibited a bond strength of
17.94 MPa. The PTPSP-epoxy-PTPSP composite structure exhibited a
bond strength of 25.75 MPa. As illustrated in FIG. 7(b), 7 days
after plasma exposure, the PTP-epoxy-PTP composite structure
exhibited a bond strength of 14.3 MPa. The PTPSP-epoxy-PTPSP
composite structure exhibited a bond strength of 24.93 MPa.
[0070] Many modifications and other embodiments of the disclosure
set forth herein will come to mind to one skilled in the art to
which these disclosed embodiments pertain having the benefit of the
teachings presented in the foregoing descriptions and the
associated drawings. Therefore, it is to be understood that
embodiments of the disclosure are not to be limited to the specific
embodiments disclosed and that modifications and other embodiments
are intended to be included within the scope of the disclosure.
Moreover, although the foregoing descriptions and the associated
drawings describe example embodiments in the context of certain
example combinations of elements and/or functions, it should be
appreciated that different combinations of elements and/or
functions may be provided by alternative embodiments without
departing from the scope of the disclosure. In this regard, for
example, different combinations of elements and/or functions than
those explicitly described above are also contemplated within the
scope of the disclosure. Although specific terms are employed
herein, they are used in a generic and descriptive sense only and
not for purposes of limitation.
[0071] It should be understood that although the terms first,
second, etc. may be used herein to describe various steps or
calculations, these steps or calculations should not be limited by
these terms. These terms are only used to distinguish one operation
or calculation from another. For example, a first calculation may
be termed a second calculation, and, similarly, a second step may
be termed a first step, without departing from the scope of this
disclosure. As used herein, the term "and/or" and the "/" symbol
includes any and all combinations of one or more of the associated
listed items.
[0072] As used herein, the singular forms "a", "an" and "the" are
intended to include the plural forms as well, unless the context
clearly indicates otherwise. It will be further understood that the
terms "comprises", "comprising", "includes", and/or "including",
when used herein, specify the presence of stated features,
integers, steps, operations, elements, and/or components, but do
not preclude the presence or addition of one or more other
features, integers, steps, operations, elements, components, and/or
groups thereof. Therefore, the terminology used herein is for the
purpose of describing particular embodiments only and is not
intended to be limiting.
* * * * *